Airplane turbojet engines are surrounded by a nacelle to protect them and ensure the operation thereof. The nacelle is made up of walls composed of non-structural panels and structural panels. The latter parts ensure a sufficient stiffness of the nacelle. To that end, structural panels usually have one or more layers of cellular core structures (commonly called “honeycomb” structures). These layers are generally covered with a skin on their so-called outer face, i.e. the face radially furthest from the axis of the engine, and on their inner face, i.e. the face radially closest to the axis of the engine.
The structural panel is then assembled by arranging the different skins and layers, which are then pasted on a mold with the required shape. The assembly is cured in a furnace so as to grip the layers and polymerize the adhesives.
In parallel, turbojet engines generate substantial noise pollution. There is therefore a strong demand aiming to reduce this pollution, and even more so given that the turbojet engines used are becoming increasingly powerful.
To that end, some of the panels used are acoustic structural panels whereof the layers are generally covered on the outer face with an air-impermeable skin, called “solid,” and on the inner face with an air-permeable perforated skin, called “acoustic.”
The structural acoustic panel can also comprise several layers of cellular core structures between which a multi-perforated skin, called a “septum,” is located. This skin is adhered between the honeycomb core structures by heating during the assembly/gluing phase of the panel.
Such panels constitute acoustic resonators able to “trap” the noise and therefore attenuate the sound emissions towards the outside of the nacelle.
In a known manner, a honeycomb core structure comprises at least one honeycomb core block comprising a central part having core honeycomb cells and two lateral parts each having a plurality of honeycomb joining cells.
The acoustic properties of the acoustic structural panel, i.e. its noise absorption rate as a function of the frequency and sound level of the noise, depend in particular on the joining of the honeycomb core block(s).
The join of the cellular joining cells is commonly done using a foaming adhesive, such as the FM410® adhesive, which has a significant expansion capacity. The adjacent edges of the honeycomb core block(s) are coated with the adhesive, which, when it expands, blocks the honeycomb cells by creating overthicknesses.
The use of adhesive requires too long a placement and cutout time of the overthicknesses from an industrial perspective.
Furthermore, these overthicknesses have the drawback of decreasing the effective acoustic surface of the honeycomb core structure as well as causing abrupt impedance interruptions, which contributes to decreasing the acoustic performance of the acoustic panel during the operation of the turbojet engine.
Also known, from application EP 1 889 713, is a honeycomb core structure whereof the honeycomb joining cells have an additional joining wall. By definition, a “honeycomb joining wall” is not engaged in the formation of the honeycomb cell. The joining is done by superimposing the additional joining walls of the honeycomb joining cells belonging to two distinct joint zones and twisting said two additional joining walls thus superimposed in a spiral.
However, such a honeycomb core structure is complex to make.